1. Field of the Invention
This invention relates to the field of grayscale representation for displays. More particularly, the present invention relates to a system and method for generating a mixed grayscale representation by presenting reduced analog gray level in multiple digital sub-frames.
2. Background of the Related Art
Conventional grayscale representations for a liquid crystal display (LCD) are generated either by an analog method that applies voltages between pixels or a digital method that adopts a time multiplexed grayscale.
Referring to FIG. 1, in the analog method, grayscale level can be generated on a screen of an analog display by varying a data voltage to modulate the brightness of each pixel. Nowadays, most displays require a grayscale of more than 8 bits per color and an operating voltage low enough to be powered by battery. This makes the voltage difference between consecutive gray levels extremely small such that the voltage difference becomes comparable to or less than the offset voltage of an analog buffer or a D/A converter used for analog signal processing, making the representation of grayscale from each D/A converter inconsistent.
Referring to FIG. 2, the digital method adopts time multiplexed grayscale in which a frame, i.e., a pixel cycle, is divided into many sub-frames, i.e., equal duration time slots. Each frame is driven ON or OFF individually. The pixel can be activated during any number of the sub-frames and the gray level is determined by the number of sub-frames which turn on. For example, the same gray level will be achieved where four sub-frames are activated whether the first four, last four or alternating sub-frames are activated.
The digital representation can be implemented in several ways by varying the sub-frame time and light intensity associated with each frame. In FIG. 3, a uniform sub-frame time and uniform illumination scheme is disclosed. The gray level represented by this method is limited by the frame-update time and liquid crystal switching time since same data is written many times to represent one bit. Generally, it is difficult to express gray level using more than 5 bits, which already require an update frequency of 5760 Hz to represent 60 images in three primary colors, red, green, and blue, as shown in Equation 1.
60(no. of images)xc3x9732(25 sub-frames)xc3x973(R,G,B)=5760 Hzxe2x80x83xe2x80x83(1) 
In another gray level representation scheme disclosed by FIG. 3, each sub-frame has weighted frame time according to the bit weight. The number of sub-frames is significantly reduced, therefore, removing the limit of updating frequency. It seems possible to represent 8 bit gray level if only updating frequency is considered. However, the shortest frame time is also limited by the frame-update time and liquid switch time as the sub-frame time disclosed by the previous scheme. Nevertheless, the update frequency required to represent 60 images with 5 bits is reduced to 900 Hz, as shown in Equation 2.
xe2x80x8360(no. of images)xc3x975(5 sub-frames)xc3x973(R,G,B)=900 Hzxe2x80x83xe2x80x83(2)
Unfortunately, this method may cause flicker since the data of the least significant bit is switched on and off in the blink of an eye and requires more complex control circuit than the uniform sub-frame time and uniform illumination scheme due to weighted frame time.
In FIG. 4, a uniform sub-frame time with weighted illumination scheme is disclosed. The sub-frame time used in this scheme is divided uniformly with weighted illumination of the light source according to the bit weight. This method reduces the complexity of control circuit and the number of sub-frames to display, e.g., it requires only 5 times of scanning for 5 bit gray level display and 900 Hz update frequency to represent 60 images, as shown in Equation 3.
60(no. of images)xc3x974(4 sub-frames)xc3x973(R,G,B)=900 Hzxe2x80x83xe2x80x83(3) 
However, this scheme has a loss of brightness as compared to a display with weighted sub-frame time and uniform illumination. For example, when all bits of 4 bit data are xe2x80x9c1xe2x80x9d, the brightness of the brightest level is calculated from the sum of brightness of each frame, where the brightness of a sub-frame is expressed by frame time times illumination. The brightest level of this method is given by
1xc3x97xc2xc+xc2xdxc3x97xc2xc+xc2xcxc3x97xc2xc+xe2x85x9xc3x97xc2xc={fraction (15/32)}≅50% 
where the brightest level of the scheme with weighted frame time is given by
{fraction (8/15)}+{fraction (4/15)}+{fraction (2/15)}+{fraction (1/15)}={fraction (15/15)}=1=100% 
As shown in FIG. 5, the brightness is almost twice of the brightness of a weighted illumination display depicted in FIG. 5. Thus, a mixed method taking advantage of both analog and digital methods is needed to decrease flicker and lower update frequency without sacrificing brightness and without complicating the control circuit.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
An object of the present invention is to provide a mixed grayscale representation that takes advantage of both the analog and digital methods by representing reduced analog gray scale levels in multiple digital sub-frames. Grayscale representation can be implemented with this mixed method without the frame frequency limitation of a digital method and the small voltage difference (xcex94V) limitation of an analog method.
For a 2D bit gray level implementation of the present invention, a frame can be divided into 2N, where N=1, 2, . . . , Dxe2x88x921. The maximum number of sub-frames is determined by Dxe2x88x921, where 2D is the number of the data bit. In an 8 bit gray level a frame can be divided into 2 sub-frames or 4 sub-frames, the sub-frame time can be either weighted or uniform. When a frame is divided into 2 sub-frames, the first sub-frame is used to display upper 4 bit data and the second sub-frame is used to display lower 4 bit data. Each 4 bit data can then be expressed in an analog manner by applying control voltages to pixel electrodes. When a frame is divided into 4 sub-frames, each sub-frame is used to display 2 bit data. Before applying the analog voltage, the voltage difference, xcex94V, between two consecutive gray levels is first determined by dividing the upper and lower voltage limits, Vpp, with the number of gray levels, G, thereby specifying the display quality for the system, i.e., xcex94V=Vpp/G. Then, the voltage corresponding to a certain gray level stated as V=xcex94Vxc3x97G is then applied to the electrodes for a whole frame to represent the desired grayscale.
In an alternative embodiment of the invention, a weighted sub-frame time can be implemented. The shortest sub-frame time in the weighted sub-frame time approach is not as short as that of a conventional digital method. An additional control circuit is used to produce weighted frame time in accordance with the weight of the 4 bit data.
In yet another embodiment of the invention, a uniform sub-frame time can be implemented. The control circuit for this embodiment can be greatly simplified with a reduction of overall brightness. Further, by making the number of sub-frames is selected to be 2N+1, N greater than 1, the brightness reduction can also be eliminated. Thus, by optimizing the these parameters, namely, the number of sub-frames and brightness, the display can be optimized. In general, the shorter sub-frame time requires a lower capacitance of memory and have a smaller liquid crystal pixel to hold the stored charge.
In yet another embodiment of the invention, two-panel display can be implemented. The first panel is used to display upper 4 bits and the second panel is used to display lower 4 bits. The intensity of light modulated by each panel is controlled by the retarder such that the intensity provided for the first panel is brighter than the intensity for the second panel by 16 times. By replacing the two temporal sub-frames used in the previous implementations with two independent panels, this implementation has reduced total frame frequency by half, allowing more flexibility in switching time of the liquid crystals.
This mixed grayscale representation method with above described advantages can be applied in most major displays that use active driving, such as TFT LCDs, liquid crystal on silicone (LCOS), electro luminescence (EL) display, plasma display panels (PDPs), field emission displays (FEDs), field sequential color display, projection displays and direct view display, such as head mounted displays (HMDs). This technique can also be used in LCOS beam deflector, phased-array beam deflector, and is especially effective in reflective displays that adopt silicon substrate backplanes.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.